DESCRIPTION: A major goal towards preventing the transmission of Human Immunodeficiency Virus type-1 (HIV-1) is the development of a broadly protective vaccine. However, this goal has been elusive due to the extreme genetic diversity of HIV-1 and poor immunogenicity of HIV-1 Env antigens. One key element of an effective protective HIV-1 vaccine is broadly neutralizing antibodies (bnAbs) that can block the infection of a diverse group of primary HIV-1 viral isolates. While bnAbs have been identified from HIV-infected patients, the problem of how to induce bnAbs through vaccination has persisted. Lately, the heterologous prime-boost approach, where the initial antigenic priming is given by a gene-based vaccine (in the form of viral vector or DNA plasmid), followed by a boost of matching Ag in the form of recombinant protein, has become a promising strategy to elicit immune protection and high quality antibody responses. Our previous research has established that a prime-boost system with HIV gp120 as the Ag is capable of inducing bnAbs in humans when the antigens were delivered as a polyvalent formulation by the DNA prime-protein boost approach. Typically, bnAbs are the product of extensive B cell clonal selection and evolution, and display a high degree of somatic hypermutation. Thus, inducing bnAbs requires a strong germinal center (GC) reaction. Because the GC reaction is dependent upon specialized follicular helper T cells (TFH cells), we investigated whether prime-boost vaccination affected the development of TFH cells. Our preliminary work revealed that DNA priming followed by protein boosting led to augmented TFH cell development, greatly enhanced GC development, and higher Ab titers and functional affinity, compared to protein immunization alone. However, the detailed immunological mechanisms controlling TFH development and activity in an HIV-1 vaccine setting are completely unknown. We have developed the hypothesis that 1) compared to protein priming, gp120 DNA priming induces a better TFH cell response, 2) careful coordination of priming and boosting immunizations affecting TFH cells and the germinal center response is crucial for the development of anti-gp120-specific antibodies at the end of prime-boost process, and 3) a stronger TFH response correlates with detectable circulating blood-TFH cells in both mice and humans. In the current proposal, we will investigate the mechanisms by which DNA priming induces TFH cells, and how this allows the host to produce high-level Ab responses following booster vaccinations. Our ultimate goal is to use this knowledge to foster the development of a highly effective HIV vaccine.